1. Field of the Invention
The present invention relates to the manufacture of magnetic disks. In particular, it relates to a process for controlling the micro-roughness of the surfaces of magnetic disks.
2. Description of Related Art
Magnetic disks are data storage devices with a very large storage capacity. For example, 400 million characters (bytes) can be stored on a magnetic disk of about 95 mm diameter. Besides the high storage density, the disks must also exhibit exact mechanical and also certain tribological properties. In subsequent practical use, the disks move at 5400 revolutions per minute. This means that the outer edge of the disk reaches a velocity of up to 100 km/h, while the read/write head is less than one ten thousandth of a millimeter (&lt;100 nm) from the surface of the disk. Only by observing the greatest precision during manufacture and by statistical process control is it possible to attain these quality requirements.
Because of its high density, a magnetic thin film medium is employed for recording the data in modem high-capacity magnetic disks. In one common arrangement, a magnetic head rests on the surface of the magnetic disk within a circumferential, data-free start-stop zone when the magnetic disk drive is not in use.
An alternative arrangement is the so-called load/unload mechanism, in which the thin film head rests outside the disk and is only guided over the disk surface when it engages in read/write activity.
As a result of the turning motion of the disk a cushion of air which supports the head is formed in both types between the head and the disk.
The conditions prevailing at the boundary layer between magnetic head and disk create a number of tribological problems. In the arrangement described, in which the head rests within the start/stop zone, the magnetic head slides over the surface of the disk until the speed of rotation of the disk is high enough to lift the head. Surface contamination, which can arise from contact of the head with the disk, can lead to abrasion of the corrosion-resistant coatings on the disk and lead to premature failure of the disk or the head.
To reduce abrasion and wear, the magnetic disk is usually provided with a lubricant. However, if the surface of the disk is essentially smooth, the high surface energy of the lubricant leads to greatly increased adhesion between the head and disk (stiction). As a consequence, the force required to turn the disk and lift the head is increased. This can easily lead to deformation and damage to the extremely delicate magnetic head suspension and hence to a failure of the whole drive.
To reduce this undesirable adhesion effect, the surface of the magnetic disk is usually roughened prior to the application of the magnetically active thin film, that is, given a texture so that the head, when it slides over the surface on which it is set down on the disk, comes into contact with very slight roughnesses (asperities) instead of with the smooth disk surface. In such a texturing process, either a number of very small grooves or valleys are ground in the magnetic disk (mechanical texturing) or deliberate elevations are produced by local fusion (laser texturing). Examples of such texturing processes can be found in U.S. Pat. Nos. 4,287,225; 4,698,251; 4,735,840 and 4,973,496.
Even with the so-called load/unload arrangement, in which the magnetic head is parked outside the disk, the texture or surface roughness of the magnetic disk plays a great part in respect of a lowest possible magnetically effective gap between magnetic head and disk.
IBM TDB Vol. 34, No. 5, pp. 381-382 describes the use of plasma processes for the manufacture of random nanostructures on a surface.
EP-A-0 567 748 discloses the manufacture and use of rough silicon surfaces. The manufacture of such surfaces with a control of the roughness density comprises a) an LPCVD (Low Pressure Chemical Vapor Deposition) process in the region of 1-5 mTorr and b) the use of a surface of thermal SiO.sub.2, which undergoes relatively little reaction with SiH.sub.4 at a temperature in the region of 500-600.degree. C. A silicon surface treated in such a way can be used as a substrate for a magnetic disk of low stiction.
Finally, Research Disclosure n289, May 1988 disclosed the evaporation of a so-called "metal undercoat" onto a substrate to achieve texturing. The evaporated metal film produces a uniform micro-roughness, so that the magnetic layer and coating subsequently applied reproduce this roughness. In a particular embodiment, chromium (Cr) is evaporated onto an aluminum-magnesium alloy substrate which supports a nickel-phosphorus surface film.
The aforementioned processes have, however, the disadvantage that they do not permit precise control of the micro-roughness of the surface and/or are very time-consuming and therefore cost-intensive.
In the manufacture of magnetic disks for desktop and server drives, the substrate predominantly used is NiP coated aluminum, to which metal atoms are then applied by sputtering. Chromium is particularly preferred as the material for the underlayer deposited on the substrate. During the sputtering process, the metal atoms grow faster on those regions of the surface of the magnetic disk which energetically favor the growth. This applies to regions which have a thicker oxide layer than their surroundings (oxide islands). Preferential oxidation takes place on regions of special topography (elevations, grooves, scratches, etc. at the atomic level) or in regions with a particular surface composition, which is thermodynamically favorable to metal growth. At these places at which the preferential growth takes place, small metal atom aggregations, the so-called nodules, are produced. These nodules are dressed by the layers (magnetic layer, protective layer) subsequently applied and result in approximately hemi-spherical protrusions on the finished disk surface.
The size and density of the nodules have a marked influence on the properties of the magnetic disk. If the nodules are relatively small and numerous (diameter 10-20 nm), the surface obtained is microscopically relatively smooth and homogeneous, allowing a very low flying height for the magnetic head and hence an efficient magnetic interaction between head and disk. However, if the head comes to rest on a disk with very small nodules, the high adhesion forces can result in an increase in stiction.
Large nodules (e.g. 50-60 nm ) with a low density of distribution, on the other hand, increase the magnetically active gap between head and disk and thus reduce the attainable write density. In addition, they produce asperities which increase the tribologically caused wear of the disk, coupled with scaling on the magnetic head, and so shorten the service life of the disk drive.
On the other hand, if the head/disk interface used tends to increased stiction, for example, through organic contamination or very smooth head surface, then large nodules, by reducing the effective contact surface of the head with the disk, act to reduce the head/disk adhesion forces.
A further negative aspect of large nodules is the danger of surface damage in the post-sputtering cleaning processes, which act abrasively on asperities and lead to damage to the topmost protective layer of the disk. This greatly reduces the corrosion resistance of the disk to the action of moisture and harmful substances.
There is therefore great interest in being able to control the size of the nodules as precisely as possible, i.e. in bringing them into a region which, on the one hand, reduces the danger of stiction and, on the other hand, minimizes the magnetically active distance in respect of the maximum possible write/read performance.